Chewbacca, like all Wookiees under the Empire, was a slave until he was rescued by Han Solo, effectively ending Solo's Imperial Navy career. Because of this rescue, he owes a "life debt" to Solo that lead to them becoming fast and faithful friends. Chewie has a reputation of being a fearsome fighter not only with his bowcaster but also with hand-to-hand combat. The "Mighty Chewbacca", as he is sometimes called, is also well versed in technical skills, and with his friend Han has turned the Millennium Falcon into a fast and powerful smuggling vessel. No one other then Han or Lando knows the ship better then Chewie does. As a pilot his skills are also excellent, and he serves as a capable co-pilot for the Falcon.
Known for his loyalties, it is very unwise to threaten someone that Chewie cares for. Even while partially restrained in the carbon freezing chamber, he managed to throw Stormtroopers around like dolls until he was successfully talked down by Han Solo. Lando himself found out just how dangerous an angry Chewie can be after he staged a breakout of the Rebels on Cloud City.